1. Technical Field
This invention relates to hetero-junction bipolar transistors, and more particularly, to double hetero-junction bipolar transistors used in power amplifiers.
2. Background Information
Hetero-junction bipolar transistors (HBTs) have been developed for high frequency power amplification systems such as mobile telephones and other mobile communication devices. While an HBT, like other transistors, includes an emitter, base, and collector, unlike conventional homojunction transistors (for example typical Si bipolar junction transistors) at least one of the emitter, base, and collector of an HBT are not formed of materials having the same bandgap. In single HBTs, the emitter and base materials are different to increase the flow of charged carriers (and thus current) in the desired direction between the emitter and base and decrease the flow of charged carriers of the opposite type in the reverse direction. A schematic of a general transistor structure is shown in FIG. 5, which shows the transistor 500, an emitter 502, a base 504, and a collector 506 as well as leads 510 from contact layers 508 on these respective portions of the transistor 500. In bipolar junction transistors, n-p-n transistors are formed using an emitter and collector doped with impurities that provide extra electrons and a base doped with impurities that provide extra holes.
In the past, the HBT has been produced primarily using gallium arsenide-based materials. Gallium arsenide-based materials are materials that include GaAs in binary, ternary or quaternary compositions. More specifically, previous HBTs include indium gallium arsenide (InGaAs) and aluminum gallium arsenide (AlGaAs). Although HBTs using these materials have been successfully incorporated into past communication devices, they remain unsatisfactory for future generation devices for a number of reasons. One of these reasons is that arsenide-based materials have relatively high thermal resistance, making them inadequate to meet the demands for the high power applications to be used in the new generation of devices, in which a large amount of heat will need to be extracted during operation.
Other HBTs have been produced using indium phosphide-based materials rather than gallium arsenide-based materials. Similar to the gallium arsenide-based HBTs, such indium phosphide-based HBTs include binary (indium phosphide i.e. InP), ternary or quaternary compositions. Indium phosphide-based HBTs have much lower thermal resistance than the gallium arsenide-based materials. Indium phosphide-based materials improve heat extraction during high power applications and exhibit the potential for high power gain in devices that require high bandwidth. However, due to the compositional structuring of the layers creating the HBTs, existing indium phosphide-based devices exhibit lower collector-base breakdown voltages than their arsenide-based material counterparts. The collector-base breakdown voltage is the maximum reverse bias voltage the collector-base diode can withstand before avalanche breakdown occurs, causing degradation of the transistor characteristics and/or destruction of the physical transistor. Avalanche breakdown is caused by the cumulative multiplication of charge carriers through electric field-induced impact ionization in the depletion region of the collector. In other words, at avalanche breakdown the electric field is large enough so that the charge carriers in the collector gain enough energy to create new pairs of electrons and holes essentially every time they collide with the atoms in the crystal structure. In HBTs used in high power applications, large electric fields are formed across the collector-base junction so that any charged carrier reaching this junction during operation is likely to undergo an avalanche-induced multiplication effect. Thus, it may be desirable to have a large collector-base breakdown voltage for high power applications.
In addition, knee voltages of HBTs used in high power applications are also of concern as these voltages are higher than desired. The knee voltage for a transistor is the point or area on a graph of emitter-base diode current versus voltage where the forward current suddenly increases. It is approximately equal to the barrier potential of the emitter-base diode. Stable operation of the HBT over a wider range of voltages may occur if the knee voltage in the HBT is decreased.
To decrease the amount of current from the collector to the base and thereby increase the gain of the transistor, double heterojunction bipolar transistors (DHBT) were developed. More specifically, InP DHBTs were designed to address many of the early challenges of current blocking. The material of the collector is different from that of the base to increase the offset between conduction and/or valence energy bands between the base and collector, to block the minority current and avoid recombination in the base region in a manner similar to the emitter-base junction. In these devices, the kinetic energies of the electrons (in n-p-n structures) entering the depletion region of the collector are far above the conduction band. Such large kinetic energies of the electrons, coupled with high electric fields, contribute to avalanche breakdown, propagating the creation of electron-hole pairs from impact ionization and causing low base-collector breakdown voltages. Thus, these design changes have not reduced the problem of high operating knee voltages and low collector-base breakdown voltages. Rather, the current DHBT structures have only served to complicate these two characteristics, thus restricting the ability of InP-based materials to realize their superior current handling capability for power applications.